1. Field of the Invention
The present invention relates to an inter-vehicle communication apparatus mounted to a vehicle to provide inter-vehicle commutation.
2. Description of Related Art
There is known an inter-vehicle communication apparatus mounted to a vehicle (i.e., subject vehicle) to provide inter-vehicle communication. The inter-vehicle communication is performed through transmitting information (e.g., positional information) about the subject vehicle to a vehicle existing around the subject vehicle, and through receiving information (e.g., positional information) about the vehicle existing around the subject vehicle. Hereinafter, the vehicle existing around the subject vehicle is also referred to as a peripheral vehicle.
JP-A-2000-90395 discloses an inter-vehicle communication apparatuses that transmits the information about the subject vehicle in a short cycle when the subject vehicle travels at a high speed, and in a long cycle when the subject vehicle travels at a low speed.
The inventors have clarified difficulties associated with the above inter-vehicle communication apparatus. The difficulties are described below, as a related art.
When one vehicle travels at a high speed and another vehicle travels at a low speed while the two vehicles are being approaching each other on a road, the inter-vehicle communication apparatus in the low-speed-traveling vehicle transmits the information in a long cycle. In the above case, the recognition of the presence of the low-speed traveling vehicle is delayed in the high-speed-traveling vehicle. The inter-vehicle communication apparatus thus cannot minimize, for example, a risk of the following accident; a subject vehicle approaches and collides with a tail vehicle of traffic-jammed vehicles that stop or travel at slow speeds due to some reasons.
When multiple vehicles travel at an approximately same high speed while forming a group, the inter-vehicle communication apparatus according to JP-A-2000-90395 has a short transmission cycle and increases communication traffic. It may thus become difficult for the apparatus in each vehicle of the group to transmit the information in a proper timing.
Referring to FIGS. 7A and 7B, the above-described points are explained below in more detail. A situation shown in FIG. 7A is such that: vehicles A, B and C are forming a group of vehicles while stopping or traveling at a low speed; and a vehicle D is approaching a tail vehicle of the group from behind the group. In the above situation, the apparatus according to JP-A-2000-90395 in each of the vehicles A, B, C is designed to have a long transmission cycle. Thus, the recognition of the presence of the vehicles A, B and C is delayed in the vehicle D. In a poor visibility condition, a risk of a probable collision cannot be eliminated.
A situation shown in FIG. 7B is such that: vehicles E to H are traveling on a same road at an approximately same speed in a same traveling direction while forming a group. When it is assumed that the vehicles E to H are traveling at a high speed (e.g., 60 km/h), the apparatus of JP-A-2000-90395 in each of the vehicles E to H has a short transmission cycle. Thus, communication traffic increases and it may become difficult for the apparatus in each vehicles E to H to transmit the information in a proper timing. The above difficulty becomes more notable as the number of vehicles in the group becomes larger.